1. Field of the Invention
The present invention generally relates to an image forming apparatus, such as a copier, a facsimile, a printer, and a monochrome and color duplicating machine, which includes an optical beam scanning device that generates a plurality of optical beams, and in particular, relates to an optical beam scanning device capable of correcting magnification of an image in a main scanning direction of the optical beam.
2. Discussion of the Background
Image forming apparatuses employing a laser beam scanning device are described, for example, in Japanese Patent Application Laid Open Nos. 9-58053 and 8-136838. The Japanese Patent Application Laid Open No. 9-58053 has, as one of its objects, obtain an image forming apparatus capable of producing a high quality image by maintaining equal magnification performance, while suppressing color deviation. A plurality of beams is generated using a plurality of laser drive circuits and laser diodes.
Each of the plurality of beams is detected by two-laser beam detecting sensors arranged at two separate positions on one main scanning line of the laser beam. The detected signals are output to a write clock generating circuit.
The write clock generating circuit counts the number of clocks responsive to the detected signal.
The number of the clocks is compared with a reference count number, and a write clock frequency is corrected and output so that the number of clocks substantially coincides with the reference count number. The write clock frequency generally controls lighting control of a laser diode, and increases image density when the number of clocks are increased.
Thus, when using such a device, a change in a scanning speed, which occasionally is caused by a change in temperature, can be corrected.
Further, Japanese Patent Application Laid Open No. 8-136838 proposes, as one of its objects to automatically correct magnification when a laser beam optical unit changes its refractive index. Accordingly magnification is responsive to a change in circumstances or similar.
To this end, a pair of light detectors is provided to detect a start and an end of scanning of a photoconductive member (hereinafter referred to as a PC member) of the laser beam.
A polygon mirror is rotated by a polygon motor that is driven under control of a polygon motor driving circuit.
A rotation speed of the polygon mirror is controlled by a magnification correcting circuit via the polygon motor driving circuit.
The control of the rotational speed of the polygon mirror is executed based on each detection signals, detected by the pair of light detectors, so that a deflection speed of the light beam between two prescribed, separate positions is constant.
The magnification correcting circuit also controls a phase of the laser beam by a laser driving circuit via a phase synchronization circuit.
A body side driving circuit controls a rotational speed of the PC member.
Further, a light beam (hereinafter referred to as a laser beam) is generally modulated by image data and is deflected at the same angular speed by a deflection device (hereinafter referred to as a polygon mirror) that rotates in a prescribed direction. The light beam, of the same angular speed deflection, is corrected by an fxcex8 lens into the same speed deflection, and executes scanning of a PC member.
However, it has been known in an image forming apparatus employing a plastic lens as a laser beam optical unit that the plastic lens changes its shape and refractive index responsive to both changes in circumstances and changes in ambient temperatures.
In addition, these changes introduce a change in a scanning position on an imaging surface of the PC member. Such a change also introduces an error of magnification in a main scanning direction, thereby resulting in a low quality image. Similarly, color deviation occurs and a color image is remarkably deteriorated due to magnification error in each color image formation.
To this end, Japanese Patent Application Laid Open Nos. 9-58053 and 8-136838 propose correcting magnification error and suppressing the color deviation occurring due to a change in the circumstances temperature or ambient temperature, respectively. In Japanese Patent Application Laid Open No. 9-58053, each laser beam of plurality of a laser beam, is detected at least at two separate positions on one main scanning line. The number of clocks generated, from when one of the laser beam detecting devices detects each of the plurality of laser beams to when the other detects each of the plurality of laser beams is counted. A write modulation frequency of each of the laser beams and the timing of the writing on a write position of each of the laser beams, which is determined from a synchronization sensor are corrected responsive to the number of clocks counted.
Thus, a high quality image can always be obtained by maintaining an equal magnification performance, while being prevented from the influence of a change in a scanning speed, which is created due to a change in the temperature.
Further, the Japanese Patent Application Laid Open No. 8-136838 proposes that a laser beam is detected at two prescribed, separate positions on a main scanning line, and a polygon mirror (i.e., a polygon motor) is controlled so that a deflection speed, detected at the two prescribed, separate positions of the laser beam, can be constant.
Thus, according to the Japanese Patent Application Laid Open No. 8-136838, magnification error in a main scanning direction can automatically be corrected (adjusted) in accordance with a change of a scanner optical unit, which is produced by a change in environment or similar.
Thus, both of the above-described conventional methods detect a laser beam at two separate positions and calculate a time difference therebetween by counting a prescribed number of clocks so as to correct (adjust) the magnification. Thus, a pair of laser beam detection sensors and a time difference calculating section are required to be provided in the two conventional devices. In addition, it has been confirmed that image magnification error occurs in the main scanning direction due to a change in a temperature of a laser beam scanning apparatus, in particular, an fxcex8 lens.
To improve accuracy of magnification correction in the main scanning direction and avoid color deviation, detection accuracy of both of the number of clocks to be counted and detection of a time difference is required to be improved. To this end, the above-described conventional methods necessarily generate the clocks to be counted at high speed (i.e., as high speed as possible, while still allowing the clocks to be counted). In such situation, even if the time difference is measured during the high speed generation of clocks, a write clock is to be changed by only a small step in order to correct a prescribed amount corresponding to one cycle (i.e., one count) of the high speed generation of clocks. The higher speed the clocks are generated at and counted, the more difficult the correction of the write clock is. In such a situation, a number of rotations of the polygon mirror can be changed by a small step.
However, even so, magnification in a sub scanning direction is changed, and color deviation occurs in a multi color image that is formed using a plurality of laser beams and plastic lenses. This forces a PC member to change its moving speed (i.e., a rotation speed) and affects an entire image formation system. In addition, in a multi color image formation, a write timing for each color is required to be changed.
Further, since an increase in a clock speed generally produces problems of stability and noise or similar, an increase in clock speed is hardly ever employed. In addition, if a write clock of an image signal is utilized as a clock to be counted, one cannot expect to obtain a higher precision than the minimum number of countable clocks.
Accordingly, an object of the present invention is to address and solve the above-noted and other problems and to provide a new image forming apparatus.
The above and other object are achieved according to the present invention by providing a novel image forming apparatus, including: a pair of light beam detecting devices that detect the light beam deflected by a deflecting device at two separate positions on a main scanning line, wherein the pair of light beam detecting devices generate both a reference time difference at a prescribed temperature and a time difference to be compared with the reference time difference in a prescribed time during image formation; a time difference measuring device that measures a time difference between time periods when the light beam is detected by one of the light beam detecting devices and when the light beam is detected by the other of the light beam detecting devices; and an image magnification correcting device that changes a write clock frequency of the light beam and the rotation number of the light deflecting device in accordance with the time difference so as to correct magnification error in the main scanning direction of the image on the image carrier.
In another embodiment, the image magnification correcting device may change the rotation number of the light beam deflecting device if magnification error is not completely corrected by changing the write clock frequency.
In yet another embodiment, an image write start position adjusting device may be provided to adjust an image write start position in the main scanning direction on the image carrier in accordance with the time difference detected by the time difference measuring device.
In yet another embodiment, an optical unit including an fxcex8 lens, and a temperature detecting device for detecting temperature of the optical unit may be included, wherein an image magnification correcting device may change the write clock frequency and the rotation number in accordance with the temperature.
In yet another embodiment, temperature of the fxcex8 lens may be directly detected.
In yet another embodiment, the time difference measuring device may measure the time difference after lowering a light beam deflection speed so as to precisely obtain a reference time difference by counting prescribed pulses.
In yet another embodiment, the time difference measuring device may measure the time difference after lowering a light beam deflection speed so as to precisely obtain a reference time difference by counting prescribed pulses.
In yet another embodiment, the light beam deflecting device may include a polygon mirror.
In yet another embodiment, the light beam deflection speed may be only lowered when the time difference is detected, and returned to a level used for image formation.
In yet another embodiment, the light beam deflection speed may be low when starting light beam deflection so as to precisely obtain a reference time difference.
In yet another embodiment, the time difference may be measured without lowering the light beam deflection speed when continuous printing is executed and time difference is detected so as to only detect needs of image magnification correction, and the magnification correction may then be executed based on a time difference detected after lowering the light beam deflection speed in a prescribed time corresponding to an interval of sheets.